A silicon carbide (SiC) power semiconductor device is formed on, for example, a solid-state single-crystal SiC substrate. As the semiconductor device, a PiN/SBD (Schottky barrier diode) mixed diode is formed. A structure of a conventional PiN/SBD mixed diode is as follows. On a surface of a high-concentration n-type (n+) silicon carbide (SiC) substrate, a low-concentration n-type (n−) SiC epitaxial growth layer is formed. A high-concentration p-type (p+) SiC region is formed in a part of a surface of the (n−) SiC epitaxial growth layer. An anode electrode configuring an ohmic junction is formed in the (p+) SiC region, and an anode electrode configuring a Schottky junction is formed on an exposed surface of the (n−) SiC epitaxial growth layer except for the (p+) SiC region. A cathode electrode configuring an ohmic junction is formed on a rear surface of the (n+) SiC substrate. In the diode, longitudinal transfer of a current between a front surface (upper surface) of the substrate and a back surface (lower or rear surface) of the substrate is used.
In a forward direction in the diode, a current begins to flow when a voltage equal to or higher than an ON voltage determined by a Schottky barrier determined by a work function difference between a semiconductor and a Schottky electrode is applied due to contribution of a Schottky junction portion. A backward direction functions such that a backward leakage current is suppressed by a depletion layer spreading from a (p+) SiC portion of a PN diode to an (n−) SiC epitaxial growth layer due to contribution of an ohmic junction portion.
The Schottky portion of the SiC device is formed by an electrode material which is Schottky-jointed between an n-type semiconductor layer and an n-type semiconductor layer. Depending on a height of Schottky barrier determined by the electrode material and a heat treatment temperature, a voltage at which a current in application of a forward voltage begins to flow is determined. However, a wide-band-gap semiconductor such as SiC has a work function higher than that of silicon (Si) used as a power device semiconductor material, and even a Schottky barrier diode has a relatively high barrier, which is a problem. When the Schottky barrier is high, an ON voltage in application of a forward voltage increases to increase a loss in energization. Furthermore, since the Schottky barrier is uniquely determined by a semiconductor material, an electrode material, and a heat treatment temperature, as long as the same material or the like is used, a desired Schottky barrier cannot be freely formed.
For this reason, various electrode materials are examined, and data is provided such that a height of the Schottky barrier can be selected in a wider range. However, a Schottky barrier having a desired height is not always obtained. Furthermore, as another problem, a decrease in barrier height for lowering ON voltage is not sufficiently examined. When a barrier height is decreased in consideration of only forward characteristics, a backward current, i.e., a leakage current also easily flows in application of a backward voltage, and the basic characteristics of a diode are not satisfied.